FIX: No Error Given for Non-static Callback Member Function

ID: Q148789

The information in this article applies to:

The C/C++ Compiler (CL.EXE), included with:
- Microsoft Visual C++, 32-bit Editions, versions 4.1, 4.2

SYMPTOMS

The compiler does not generate any compiler errors when a non-static member function is used as a callback function and the calling convention for the function is explicitly specified. For example, the sample code shown in this article will not cause the compiler to generate any compiler errors although it should.

Debugging the non-static member callback function will reveal the stack is corrupted. Any parameters to the function, including the implied this pointer may be invalid.

RESOLUTION

By design, a member function used as a callback function must be static. A non-static member function cannot be used as a callback function. You can also use file scope global functions for callback functions.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This bug was corrected in Visual C++ version 5.0.

MORE INFORMATION

Using a non-static member function as a callback function should trigger the compiler to generate the following error when the callback function is invoked:

test.cpp(line#): error C2643: illegal cast from pointer to member.

In C++ every callback function has its own prototype, which determines the parameters that the calling function can pass to the function. However, C++ member functions have a hidden parameter known as the 'this' pointer. C++ is able to associate a function with a particular instance of an object by means of the 'this' pointer. Therefore, if a callback function, defined as a non-static member function, is called, it will expect a particular instance of an object. On the first line of the Test::Test_Func() member function in the following sample code, there is no object instance when the callback function is invoked, so a C2643 compiler error should be generated.

For more information on writing C++ callback functions, please see the article "Calling All Members: Member Functions as Callbacks" on the Microsoft Development Network (MSDN) CD-ROM.

Sample Code to Reproduce Problem


   /* Compile options needed: default console application
   */ 
   #include <stdio.h>
   typedef void (__cdecl *lpCallback)();
   void Function1(lpCallback)
   {
   }
   class Test
   {
    public:
      void _cdecl BadCallBack();       // Non-static callback function
      static void _cdecl GoodCallBack(); // Static callback function
      void Test_Func();
   };
   void Test::Test_Func()
   {
     Function1(BadCallBack); // Line 20 : compiler doesn't generate C2643
     Function1(GoodCallBack);
   }
   void Test_Func()
   {
     Function1(Test::BadCallBack);  // Compiler correctly generates C2643
     Function1(Test::GoodCallBack);
   }
   void main()
   {
   }

Additional query words:

Keywords : kbcode kbprg kbCompiler kbVC410bug kbVC420bug kbVC500fix
Version : 4.1 4.2
Platform : NT WINDOWS
Issue type : kbbug